Plant Pathology and Disease Management Lecture Notes

Vocabulary terms and academic definitions covering the fundamental concepts of plant pathology, specific crop diseases, and management strategies including biological and chemical control.

Plant Disease

A dynamic, progressive deviation from the normal physiological and morphological state of a plant, caused by a continuous irritation by a primary causal agent, resulting in suffering (pathos) and manifested in characteristic symptoms.

Injury

An instantaneous event that disrupts plant health, such as a tractor scrape or insect bite, as opposed to a disease which is a process occurring over time.

Disease Triangle

The conceptual model where disease establishment requires the simultaneous interaction of a susceptible Host, a virulent Pathogen, and a favorable Environment.

Vascular Wilt Pathogens

Pathogens such as Fusarium and Verticillium that clog xylem vessels with mycelium, spores, or induced gums and tyloses, disrupting the translocation of water and nutrients.

Obligate Parasites (Virus Mechanism)

Pathogens that hijack the host's nuclear machinery to force the plant to synthesize viral proteins and nucleic acids instead of its own functional proteins (transcription and translation disruption).

Viroids

Sub-microscopic, naked, single-stranded RNA molecules that act as plant pathogens, such as the Potato Spindle Tuber Viroid.

Phytoplasmas

Wall-less prokaryotes that inhabit phloem sieve tubes and cause symptoms like phyllody and witch's broom.

Oomycetes

Water molds like Phytophthora that possess cellulosic cell walls and cause diseases such as rusts, smuts, and mildews.

Endemic Disease

A disease constantly present in a moderate to severe form in a particular geographic area, such as Wart disease of potato in Darjeeling.

Epidemic (Epiphytotic)

A disease that occurs widely but periodically in a severe, destructive form over a vast area, such as Wheat stem rust.

Inoculum Potential

The energy and quantity of pathogen propagules (spores, sclerotia, bacterial cells) available per unit area.

Monocyclic Epidemics

Single-cycle diseases (often soil-borne or seed-borne) where the pathogen has only one infection cycle per crop season and follows a linear growth curve.

Polycyclic Epidemics

Multi-cycle diseases (often air-borne) where the pathogen completes multiple reproductive cycles within a single season, following an exponential or Sigmoid (S-shaped) curve.

Van der Plank Formula

$\frac{dy}{dt} = r \times y_t (1 - y_t)$ where $y$ is the fraction of diseased tissue, $t$ is time, and $r$ is the infection rate.

Irish Potato Famine (1845)

A historical epiphytotic caused by Phytophthora infestans that led to widespread starvation and demographic shifts.

Koch's Postulates

A four-step scientific protocol used to prove a specific microorganism is the causal agent of a disease: Constant Association, Isolation in Pure Culture, Re-Inoculation/Symptom Reproduction, and Re-Isolation.

Tyloses

Induced structural defenses consisting of balloon-like overgrowths of parenchyma cells that plug xylem vessels to block the migration of wilt pathogens.

Hypersensitive Response (HR)

A localized, deliberate suicide of host cells immediately surrounding the infection site to cut off the food supply for biotrophic pathogens.

Phytoalexins

Low-molecular-weight, antimicrobial toxic compounds synthesized rapidly by plants only around infected cells, such as Pisatin in peas.

Systemic Acquired Resistance (SAR)

A defense state primed throughout the entire plant by signal molecules (primarily Salicylic Acid) following a localized infection.

Peronosclerospora sorghi

The causal agent of Downy Mildew of Maize, characterized by coenocytic mycelium and systemic chlorotic striping.

Green Ear Disease

A phase of Downy Mildew of Pearl Millet (Sclerospora graminicola) where floral parts are transformed into green, leafy structures.

Puccinia graminis f. sp. tritici

A macrocyclic and heteroecious rust fungus that causes Black/Stem Rust of wheat, requiring Barberry (Berberis vulgaris) as an alternate host.

Ustilago nuda f. sp. tritici

An internally seed-borne pathogen causing Loose Smut of wheat, which transforms the inflorescence into a powdery mass of olive-black spores.

Solar Heat Treatment (Luthra & Sattar)

A management method for Loose Smut of wheat involving soaking seeds in water for 4 hours followed by 4 hours of sun drying at $40\text{--}45^\circ\text{C}$.

Bipolaris oryzae

The pathogen causing Brown Spot of rice, historically linked to the Great Bengal Famine of 1943.

Tikka Disease

A fungal leaf spot disease of Groundnut caused by Cercospora arachidicola (Early) and Cercosporidium personatum (Late).

Xanthomonas axonopodis pv. citri

The Gram-negative, rod-shaped bacterium that causes Citrus Canker and produces yellow-colored colonies due to xanthomonadin pigment.

Pectobacterium carotovorum

A facultative anaerobic bacterium with peritrichous flagella that causes Soft Rot by secreting pectolytic enzymes to liquefy plant tissue.

Leifsonia xyli subsp. xyli

A small, non-motile vascular bacterium that causes Ratoon Stunting Disease (RSD) in sugarcane, characterized by reddish-brown, comma-shaped streaks in vascular bundles.

Phyllody

A floral malformation where floral parts turn into green leafy structures, common in Little Leaf of Brinjal caused by Phytoplasma.

Six Principles of Disease Management

The core framework of plant pathology control: Avoidance, Exclusion, Eradication, Protection, Resistance, and Therapy.

DIPA (1914)

The Destructive Insects and Pests Act, a plant quarantine law in India used for the exclusion of pathogens.

Soil Solarization

A physical eradication method using transparent polyethylene sheets to trap solar energy, raising soil temperatures to $50\text{--}55^\circ\text{C}$ to kill soil-borne pathogens.

Systemic Fungicides

Curative chemicals absorbed by plant tissues and translocated via the vascular system (e.g., Carbendazim, Metalaxyl).

Dithiocarbamates

A class of organic synthetic fungicides including Mancozeb, Zineb, and Thiram.

Surfactants (Wetting Agents)

Additives that lower the surface tension of water drops to prevent them from rolling off waxy leaf cuticles.

Biological Control Agent (BCA)

A living organism, such as Trichoderma harzianum or Pseudomonas fluorescens, used to inhibit plant pathogens.

Antibiosis

A biocontrol mechanism where the antagonist inhibits the pathogen by secreting toxic metabolites or antibiotics like DAPG or HCN.

Siderophores

Low-molecular-weight, iron-chelating compounds produced by bacteria like Pseudomonas fluorescens to starve pathogens of ferric iron ($Fe^{3+}$).

Integrated Disease Management (IDM)

A holistic approach combining cultural, biological, physical, genetic, and chemical practices to keep pathogen populations below the Economic Injury Level (EIL).